Field of the Invention
The invention concerns a method for generating 4D flow images by operation of a magnetic resonance system and a device for generating 4D flow images by operation of a magnetic resonance system. The invention also concerns such a magnetic resonance system.
Description of the Prior Art
MR phase contrast flow imaging (also known as “phase contrast flow imaging”) is a non-invasive quantitative method which is usable in vivo. Phase contrast flow imaging is based on the different phases that can develop during an MR recording under transverse magnetization. In the presence of time-varied gradient fields, a moving magnetization in the outer B0 field accumulates one phase. This can be used to encode velocities. For this purpose, the different influences of the MR imaging on the magnetization is compensated for by special gradient switching sequences such that only phase differences which form due to the movement of the magnetization remain, for example, during the flow of blood in a vessel relative to the stationary vessel wall. Otherwise expressed, due to the application of a bipolar gradient, the magnetization of the flowing material attains a phase offset which is proportional to the velocity of the flowing particles. Static examination regions ideally do not have this phase. The phase information of an MR signal is contained in the imaginary portion of the signal which, in conventional MR scans is typically discarded or is set against the actual anatomical magnitude image. In the normal case, the imaginary portion of the signal does not contain any meaningfully useful information. However, during a phase contrast scan, the imaginary portion is reconstructed, as a separate data record, into a phase image. This phase image contains encoded in each image point as a gray scale value the information concerning the velocity and direction of the corresponding image point or image voxel. The direction is encoded as a gray scale. At white image points, the flow is directed toward the observer and at black image points, the flow is directed away from the observer.
In the phase contrast flow scan, different interference phenomena which can impair the result arise. One phenomenon is “phase wrap-around” or the aliasing that is attributable thereto. This is based on the fact that only values of the phase of the transverse magnetization from the +180° to the −180° directions are correctly recognized. All values beyond this are not correctly recognized and are erroneously represented as flows with the opposite direction in the data record or in the phase image. Therefore, before a phase contrast scan, the velocity range of the region to be investigated or of the fluids moving therein must be specified. The gradient profile of the sequence is modified such that the maximum phase difference of +−180° corresponds to the velocity range given. This velocity is denoted “encoding velocity” (VENC). Values of VENC that are too low lead to the aforementioned aliasing. Conversely, the noise of phase contrast flow imaging increases with the height of the VENC so that with the VENC selected to be too high, falsification of the measurement results can also be expected.
Further errors arise from phenomena which occur due to eddy currents and Maxwell terms (magnetic fields of high order). For this reason, in a phase contrast flow imaging scan, the phase of the static regions is also not exactly zero. In order to compensate for the aforementioned errors, the Maxwell terms can be calculated and the images can be corrected with the computation results. However, the eddy currents are much more difficult to correct or compensate for, since they are very difficult to predict.
Recently, a new type of phase contrast imaging called “4D flow” has been developed with which a complete vectorial measurement of flow fields in one volume can be achieved. Herein, velocity information over time is generated in a three-dimensional space. The recording time of a 4D flow data record is typically very long due to the multiple velocity encodings in different directions and a spatial and temporal coverage of the region to be investigated or the required spatial or temporal resolution associated therewith. Furthermore, a navigator is typically used in order to enable clocking of the recording with the breathing rhythm during a long-duration recording time. In this way, recording times of approximately 15 minutes are achieved.
Compared with the established phase contrast flow imaging for the measurement of the flow velocities, wherein a single slice is recorded with a flow encoding perpendicular to the slice plane, in the “4D flow” sequence, data are recorded with a plurality of flow encoding directions (flow-compensated (resultant first magnetic moment M1=0), vx, vy, vz). Typically, the innermost loop in the sequence is designed such that data for a number of different flow encoding directions can be recorded therein. For recording the vector field, bipolar gradients are applied. Since the bipolar gradients for different flow encoding directions are applied in different directions, the effectively repeated gradient pattern which gives rise to the eddy currents in the stationary state is repeated during the recording of a 4D flow protocol in a time interval which corresponds to the effective repetition time TR(4D)=4×ES, wherein ES describes the echo spacing of the underlying imaging sequence, which repeats according to the number of velocity encoding directions as compared with simple phase contrast flow imaging. For this reason, the eddy currents arising in equilibrium (after many repetitions of the gradient pattern) are possibly stronger and the background phase connected thereto with “4D flow” is higher than with flow encoder protocols with a single velocity direction where the same velocity encoding gradient is applied at the time offset of a shorter repetition time TR(2D)=2×ES.
Furthermore, there are measures with which a more rapid image recording is ensured wherein the overall recording time can be achieved in the order of a few minutes, making use of a classic navigator.
Furthermore, improved movement registration techniques are also available, permitting registration of data records over a plurality of breathing cycles. The acceleration of the 4D flow technology to a duration of a few breaths would enable a clinical application.
However, the conventional procedure for 4D flow image recordings results in the following difficulties. Conventionally, for 4D flow image recording, the velocity encoding is carried out in the innermost loop, while the patient table is stationary. Due to the rapid gradient changes, eddy currents are formed, resulting in a background signal which intensifies with distance from the isocenter.
Furthermore, the following problems exist, which make a clinical application of the 4D flow method more difficult. Firstly, the recording times for “4D flow” are still too long and, secondly, more intense eddy currents occur, particularly with an accelerated procedure, due to the more complex flow encoding scheme and the different flow encoding directions for “4D flow”.